Coping
with temperature fluctuation
Most
fishes are ectothermic, so their body temperature reflects that of the
surrounding environment. Fishes that experience changing environmental
temperatures, such as those characteristic of diel or seasonal changes, have
several cellular and subcellular mechanisms for adapting to the newest of
conditions. Many physiological adjustments are the result of switching on or
off genes that are responsible for the manufacture of particular proteins. For
example, acute heat stress initiates the synthesis of stress proteins, also
known as heat shock proteins or HSPs, which maintain the
structural integrity of proteins that otherwise would become denatured at
higher temperatures, thereby allowing them to function biochemically.
To
compensate for the decreased rate of biochemical reactions at low temperatures,
fishes may increase the concentration of intracellular enzymes by altering the
rate of enzyme synthesis, degradation, or both. Increased cytochromec
concentration in Green Sunfish (Centrarchidae)that were moved from 25 to 5°C is
due to a greater reduction in the degradation rate than in the rate of
synthesis(Sidell 1977).
In some
fishes alternative enzymes (termed isozymes)may be produced to catalyze
the same reaction more efficiently at different temperatures. Isozymes are
regulated by switching on or off the different genes that control their
production. Rainbow Trout (Salmonidae) acclimated to 2 versus 18°C exhibit
different forms of acetylcholinesterase,an enzyme important to proper nerve
function because it breaks down the neurotransmitter acetylcholine(Hochachka
& Somero 1984). The ability of Long jaw Mud suckers (Gobiidae) to tolerate
rather wide ranges of temperature is probably due to the fish’s ability to
regulate the ratio of isozymes of cytosolic malate dehydrogenase, an important
enzyme in the Kreb’s cycle (Lin & Somero1995).
Polyploid
species have extra sets of chromosomes (see Polyploidization and evolution),
and may havea better capacity to cope with a wide range of temperatures;
perhaps the multiple copies of genes provide more opportunitiesfor evolution to
bring about changes in alleles that may prove to be beneficial. For example,
among cyprinids, Goldfish and Common Carp are both polyploid and can tolerate a
wide range of temperatures, and the polyploidy Barbel can acclimate better to
different temperatures than can the diploid Tinfoil Barb (O’Steen & Bennett
2003).
Laboratory acclimation studies, in which a single variable such as temperature is altered while other factors are controlled and remain constant, can be helpful in understanding how fishes respond to a change in a single variable. However, in their natural habitats, fishes usually acclimatize to simultaneous changes in several variables, such as temperature, photoperiod, and perhaps reproductive condition as seasons change. The absence of natural seasonal cues, such as changing photoperiod, may cause an artificiallyacclimated fish to respond somewhat differently than one that has been naturally acclimatized. For example, laboratory acclimated fishes typically have higher metabolic rates at higher temperatures (see Metabolic rate), yet seasonal reproductive cycles cause naturally acclimatized sunfish (Centrarchidae) to have higher metabolic rates in spring than in summer (Roberts 1964; Burns1975). Other studies also have shown seasonal changes in metabolic rate that were independent of temperature in trout (Salmonidae; Dickson & Kramer 1971), two minnows (Cyprinidae; Facey & Grossman 1990), sunfish(Evans 1984), and sculpin (Cottidae; Facey & Grossman1990).
Some
fishes exhibit allozymes, alternative forms of the same enzyme that are
controlled by different alleles ofthe same gene. Different populations of the
species may exhibit higher or lower frequencies of the appropriate alleles
depending on their geographic location. Livers ofMummichog (Cyprinodontidae)
along the east coast of the United States exhibit two allozymes of lactate
dehydrogenase, an important enzyme in carbohydrate metabolism. In Maine, the
frequency of the allele for the form more effective at colder temperatures is
nearly 100%, and the frequency decreases progressively in populations further
to the south (Place & Powers 1979). In Florida, the alternative allele,
which codes for the form more effective at higher temperatures, has a frequency
approaching 100%.
Acclimation
to cold temperatures includes modification sat the cellular and tissue level as
well. Fishes, as well asother organisms, can alter the ratio of saturated and
unsaturated fatty acids in their cell membranes to maintain uniformity in
membrane consistency (Crockett & Londraville2006). The proportion of unsaturated
fatty acids, which are more fluid at colder temperatures (e.g., compare
vegetable oil and butter at low temperature), increases in those species that
are active during winter. Some fishes also decrease cholesterol levels in cell
membranes to increase fluidity at lower temperatures. Fishes that live in very
cold habitats, such as polar seas (see Polar regions), often show cellular-level
metabolic adaptations such as enzymes that function well at low temperatures
and more mitochondria in their swimming muscles (Crockett &
Londraville2006). Therefore, they can function better at lower temperatures
than would a nonpolar fish acclimated to very low temperature.
Decreased
muscle performance at low temperatures can be compensated for at several levels
of muscle function.
Acclimation
of Striped Bass (Moronidae) to low temperatures results in a substantial
increase in the percent of red muscle cell volume occupied by mitochondria
(Egging ton& Sidell 1989), and an overall increase in the proportion of the
trunk musculature occupied by red fibers (Jones &Sidell 1982); both of
these adaptations would increase the aerobic capability of the fish. Muscle fibers
of Goldfish(Cyprinidae) show an increased area of sarcoplasmic reticulum at
lower temperatures (Penney &Golds pink 1980),which would make available
more calcium ions to help activate more proteins needed for contraction.
At colder
temperatures fishes may utilize more muscle fibers to swim at a particular
speed than they use at warmertemperatures (Sidell &Moreland 1989). Because
lower temperatures require the recruitment of more muscle fi bersto sustain a
given speed than is necessary at higher temperatures, maximum sustainable
swimming velocities are lower at low temperatures (Rome 1990).
Temperature
changes may affect ion exchange at the gills in a few different ways (Crockett
& Londraville 2006).
Higher
temperatures typically increase molecular activity, causing increases in
diffusion rates. Changes in membrane fluidity due to changes in the saturation
of fatty acids or concentration of cholesterol, as discussed earlier, can also
affect membrane permeability – less fluid membranes tend to be more permeable.
Freshwater fishes often show increased activity of Na-K
adenosinetriphosphatase(ATPase) at lower temperatures, whereas marine fishes
show increased Na-K ATPase activity at higher temperatures. Both trends suggest
increased metabolic activity to maintain osmotic balance as temperature
changes.
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